We have isolated recombinant clones of human cellular DNA, using low stringency blot hybridization, which contain mouse mammary tumor virus (MMTV) related sequences. The major region of homology corresponds to a 0.9 kbp restriction fragment containing the junction of the MMTV pol and env genes. Using these hybridization conditions MMTV and mammalian type C proviral DNA sequences do not hybridize. Similar comparisons of infectious murine type A and Squirrel monkey retroviruses (SMRV) type D proviral DNA have shown that each contains a major homology region with the MMTV Pol gene. This DNA sequence homology is consistent with the similarity between the respective reverse transcriptase divalent cation requirement as compared to that of the mammalian type C polymerase. The MMTV related human recombinant clone HLM-2 also hybridizes with the pol gene of Type A and D proviral genomes. In addition, HLM-2 hybridizes with the LTR and gag regions of the SMRV proviral genome. The organization of these retroviral related sequences in HLM-2, as well as, four additional independent human recombinant clones is consistent with the existence of a novel family of genetically transmitted proviral genomes in human cellular DNA. These sequences are distinct from human type C related sequences. There are 20-30 copies of these sequences in human cellular DNA. Analysis of human-mouse somatic cell hybrids suggests that they are located on four chromosomes. Comparison of restricted cellular DNA from several normal tissues and breast tumors has revealed no major amplification or variation in the pattern fragments containing related sequences. One exception is the breast tumor tissue culture cell line MCF-7 which contains at least two new MMTV related restriction fragments. These sequences do not appear to be completely homologous to the novel class of human retroviral related sequences.